1. Field of the Invention
The present invention relates to a fuse, and in particular to a cartridge-type fuse and a manufacturing method therefor.
2. Description of the Prior Art
In the field of cartridge fuses there is one particularly interesting prior art fuse disclosed in Japanese Laid-Open Utility Model Publication No. 59-66844, in which a high-melting-point, fusible metal conductor, such as a copper or tinned copper conductor, is provided with a low-melting-point metal chip, such as a tin chip. In this fuse, the low-melting-point chip is selected so as to melt and fuse to the high-melting-point conductor to form an alloy therewith when the electric current passing through the metal conductor exceeds the predetermined rating of the fuse. The end result is that the alloy, having a relatively high electrical resistance, will either rupture or melt due to the excessive electric current.
With reference to FIGS. 1 and 2, there is shown a specific example of the type of fuse mentioned above. Namely, as shown in FIG. 1, a fuse element 1 comprises a metal conductor 2 having a recess 2a and a clamping strip 2b at a central portion thereof for holding a tin chip 3. The metal conductor 2 is further provided with welding portions 2c and stoppers 2d formed at both ends thereof.
Next, with reference to FIG. 2, a fuse 5 is shown comprising the fuse element 1 and terminal fittings 4 which are to be welded to the fuse element 1. As illustrated, the fuse element 1 is shown in a state in which the clamping strip 2b is bent around the tin chip 3, and the terminal fittings are shown comprising base portions 4a, notches 4c for receiving the stoppers 2d of the metal conductor 2, and welding surfaces 4b for receiving the welding portions 2c to be welded therewith. Finally, even though it is not shown in FIGS. 1 and 2, the fuse 5 is partially encased in protective insulation.
Besides the example just given, it is also possible to construct the same type of cartridge fuse by forming the metal conductor and terminal fittings together as a single element, and thus avoid the step of welding. Moreover, it is also possible to replace the recess in the metal conductor with a plurality of lateral grooves in order to improve the contact between the tin chip and the metal conductor, as suggested by Japanese Laid-Open Utility Model Publication No. 62-1349.
Unfortunately, however, all these prior art cartridge fuses have the disadvantage that their response times, i.e., the time it takes for the fuse element to either melt or rupture, due to excessive electrical current flow, increase long periods of use. As for why these decays in response time occur, extensive research has revealed that the tin chip becomes oxidized over long periods of use, especially when used in environments that are fairly high in temperature. This oxidation then leads to a direct slowing down in the chip's ability to melt and fuse with the high-melting-point metal conductor.